The field of the invention is patient monitoring systems. More particularly, the invention relates to a patient monitoring method and system that determines blood pressure and pulse rate.
The heart muscles of humans periodically contract to force blood through the arteries. As a result of this pumping action, pressure pulses exist in these arteries and cause them to cyclically change volume. The maximum pressure that occurs at a location in an artery during a heart cycle is known as the systolic pressure and the minimum as diastolic pressure. These volume changes can be used to estimate oscillometric blood pressure values when measured with an applied pressurized cuff wrapped around the limb of a patient. From this information the oscillometric blood pressure is derived. Additionally, during the determination of an oscillometric blood pressure, it is desirable to make an estimate of the heart rate. If pulse period timing is gathered during the blood pressure determination, the pulse rate can be estimated.
There are different techniques and devices for measuring blood pressure and pulse rate values. One method in particular involves applying an inflatable cuff around an extremity of a patient's body, such as the patient's upper arm. The cuff is inflated to a pressure above the patient's systolic pressure and then reduced over time while a pressure sensor continues to measure the cuff pressure. The sensitivity of the sensor is such that pressure fluctuations within the cuff resulting from the beats of the patient's heart may be detected. With each beat there is a resulting small change in the artery volume which is transferred to the inflated cuff causing slight pressure variations within the cuff which are detected by the pressure sensor. The pressure sensor produces an electrical signal showing the cuff pressure and a series of small periodic variations associated with the beats of a patient's heart. These variations, called “complexes” or “oscillations,” are used to determine the patient's blood pressure and pulse rate. This method of blood pressure and pulse rate determination is generally known as the oscillometric method.
Blood pressure measuring devices operating according to the oscillometric method detect the peak-to-peak amplitude of the pressure complexes at various applied cuff pressure levels. The amplitudes of these complexes, as well as the applied cuff pressure, are stored together as the device automatically changes the cuff pressures over a range of interest. The time period between the oscillations are often “filtered” or “averaged” to determine the pulse rate.
The reliability and repeatability of pulse rate computations hinges on the ability to accurately determine the complexes and their associated time period. Unfortunately, there are many barriers to accurately and reliably detecting and measuring oscillation characteristics, particularly the time period between the oscillations. For example, patient motion, vibrations, and other interference may cause artifacts in the pressure signal obtained from the cuff during blood pressure determinations. These artifacts are superimposed upon the desired oscillation signal, causing it to be distorted and making any timing measurements unreliable. It is often difficult to get two consecutive artifact free pulses. Furthermore, if the pulses are non-consecutive, determining the appropriate pulse period can be especially difficult. In order to provide greater patient comfort it is often desirable for blood pressure algorithms to take very little data at a single pressure step. Typically, oscillometric algorithms gather matched pulses at each pressure step. However, to accelerate a determination these same algorithms will change mode and take only a single pulse at some pressure steps. This forces these algorithms to try to compute pulse periods across pressure deflation steps. Oscillometric blood pressure determinations gather oscillations that are of different sizes depending on the applied cuff pressure. At times these pulse amplitudes may be so small compared to the artifact level that it is very difficult to get uncorrupted complexes and their associated timing. Pulse periods computed on low amplitude signals may output an inaccurate pulse rate. For these reasons, it is difficult to decide whether to publish blood pressure and pulse rate results or provide an indication that an accurate result is not possible with the given set of data. Therefore, there exists a need for a system and method to determine the quality arid reliability of measured blood pressure data prior to making final determinations of a patient's actual blood pressure and pulse rate.